While most molecules have paired electrons in consequence of covalent bonding, some molecules—including free radicals—have electrons that are not paired. Paired electrons have opposite spins (Ms=+/−½) that cancel out net magnetic moments. Unpaired electrons have spins that can interact with magnetic fields.
Unpaired electrons in molecules will resonate in a magnetic field. Electron Paramagnetic Resonance Spectroscopy (EPR), sometimes known as Electron Spin Resonance Spectroscopy, takes advantage of this effect to quantify and determine environments of the unpaired electrons. This is done by applying a magnetic field to a substance, which may be located within a human or animal subject, to align spins of any unpaired electrons in the substance. Once spins are aligned, a response of the spins of the unpaired electrons in the substance to radio-frequency electromagnetic radiation at and near a resonant frequency is measured. The resonant frequency and amount of absorption of the electromagnetic radiation is often dependent on the local environment of the unpaired electrons in the molecule as well as the applied magnetic field. The resonance results in such effects as a spike in a radio-frequency absorption spectrum of the substance in a magnetic field.
An EPR spectrum is often acquired by placing a sample in a magnetic field, holding a frequency of a radio frequency source and absorption measuring device constant, and making repeated measurements of response of the sample to the radio frequency energy while sweeping the intensity of the magnetic field. An EPR spectrum may also be obtained by repeated measurements of absorption made while holding the magnetic field intensity constant and sweeping the frequency of a radio frequency source and measuring device. The measuring device may measure spin echoes in addition to, or in place of, absorption.
Unpaired electrons are naturally found in small quantities in chemicals, such as free radicals, that are found in biological materials. For example, free radicals are produced during, and have importance in oxidative energy production by mitochondria. The amounts of these free radicals, however, are very low in unirradiated teeth and therefore will not contribute to the EPR signal detected in vivo
It is known that certain hard tissues, including the hydroxyapatite in tooth enamel and keratin in fingernails, develop and retain unpaired electrons capable of producing an EPR signal when teeth and/or fingernails are subjected to ionizing radiation.
In the case of teeth, this EPR signal is roughly proportional to the mass of tooth enamel and to the total radiation dose received in that mass since the tooth formed. This radiation-induced signal has a long half-life on the order of hundreds of thousands of years.
Nuclear accidents resulting in significant radiation exposures to workers in the nuclear industry are known; for this reason many such workers carry film badges or dosimeters with which to determine their exposure in any incident. Occasionally, such as the 1945 death of Harry Dahglian, or in 1946 the death of Lewis Slotin (both exposed to the same plutonium bomb pit, which had gone critical due to erroneously placed, nearby, neutron reflecting materials), such dosimeters have been left elsewhere during an incident, or the radiation dose may exceed the range of the dosimeter—estimates of Slotin's exposure are as high as 9-11 Gray.
While nuclear reactor operators and other workers in the nuclear power and medical radiation treatment industries typically carry dosimeters for measuring radiation exposure in their work environment, members of the public, emergency services crews, and armed forces rarely carry such dosimeters. In the event of nuclear accident, terrorism, or warfare, it would be desirable to measure recent radiation exposure of people exposed to such events, including those who do not habitually carry dosimeters.
Past techniques for measuring EPR signal in teeth to determine radiation dose have required extraction of a tooth, a procedure that would not be logistically feasible with the expected victims of nuclear disasters.
In an international climate where perpetual enemies India and Pakistan are both nuclear powers, where North Korea has nuclear weapons and Iran—a country that has threatened Israel, a country widely believed to be a nuclear power—may soon acquire them, and organizations such as ISIS have threatened to smuggle and detonate either a nuclear device or a “dirty bomb”, the risk of a nuclear attack or terrorism is rising. Further, with worldwide interest in nuclear power to produce electricity without emitting greenhouse gasses, there is a significant risk of nuclear accident, possibly involving off-site release of contaminants as with Chernobyl or Fukoshima. Nuclear accidents have also resulted from improper disposal of radioactive materials, such as radiation treatment machines.
In a nuclear attack, nuclear improper-disposal events, and nuclear accidents, there may be people potentially exposed to ionizing radiation while not carrying previously-issued dosimeters. Both a nuclear attack and a nuclear accident could be mass-exposure situations with several hundred to tens of thousands of people potentially exposed to radiation.
The Chernobyl, Goiânia, and Hiroshima events each involved at least some deaths from acute radiation syndrome, as have other events. Gabriella, the Brazilian scrap-metal scavenger who carried the glowing core of a radiation treatment machine across a city in a bus, not only gave herself a fatal radiation dose, but exposed others to varying and unknown doses of radiation, as did her husband and others in the family These events illustrate need for measurements of radiation exposure in potentially thousands of individuals of widely varying radiation exposure.
In mass exposure situations there are widespread serious concerns, where ‘worried-well’ people physically unaffected by the event may believe that their lives are in danger and therefore require methods to determine objectively that they do not have life-threatening exposures to ionizing radiation. This phenomenon is expected to occur in nuclear events such as nuclear attack, terrorism, or accident. Further, there are likely to be limited medical facilities available after some such events—treatment of everyone, the ‘worried well’ as well as the exposed, is not expected to be possible immediately after a major event.
It is desirable to be able to rapidly sort people into categories which may include:                those who are ‘worried well,’ needing no treatment;        those with minimal exposure-possibly sufficient to cause increased cancer rates or otherwise need followup-but who will not need immediate treatment for acute radiation sickness;        those who have received significant exposure but should recover from acute radiation sickness with conventional therapy such as antibiotics and transfusions;        those who should recover from acute radiation sickness with aggressive therapy such as marrow transplants; and        those who will probably die regardless of treatment.In the short term, treatment can then be focused upon those groups who most likely will benefit from the treatment. The process of sorting people according to injuries into treatable, urgently treatable, or untreatable groups is known as triage        
Existing technologies for determining those exposed to large doses of radiation are often based on biological responses to ionizing radiation and include changes in white blood cells, changes in levels of gene expression, changes in metabolites and changes in proteins produced. Unfortunately, not only do such measurements require skilled medical staff, but baseline levels vary among individuals, responses to radiations vary among individuals, these levels are likely to be dependent on both prior history and acute simultaneous events such as concurrent stress and trauma. Also, the biologically-based changes must vary over time as they depend on naturally occurring reactions to damage and therefore must have time-dependent changes as the responses are indicated, modulated, and then tend to return towards normal levels. Physically based biodosimetry, specifically electron paramagnetic resonance measurements of radiation-induce free radicals in the enamel of teeth, differ from the biologically based responses as being independent of prior history (except for therapeutic irradiations that include high doses to the teeth), independent of concurrent stress and trauma, and have an immediate and unchanging dose-dependent response to ionizing radiation that can be rigorously quantitated.